1. Field of the Invention
The invention relates to the blending of particulate solids and in particular to a method and apparatus for converting a heterogeneous mixture of fine and cohesive UO.sub.2 powders into a homogeneous mixture.
2. Description of the Prior Art
The blending of particulate solids has been accomplished in the past in a variety of ways. Mechanical mixers of several types, such as tumble mixers, ribbon blenders and high shear mixers, have been used, as well as so-called bubbling-bed fluidized bed blenders and spouting bed blenders. In the prior art, UO.sub.2 powders have primarily been blended with mechanical tumble-type blenders, such as disclosed in U.S. Pat. No. 3,825,230 to Frye et al. However, problems attendant upon the use of mechanical tumble blenders in general and in particular with the use of mechanical tumble blenders for the blending of UO.sub.2 powders brought the development of the bubbling-bed fluidized bed blenders.
The aforementioned Zenz et al application discloses an improved bubbling-bed fluidized bed blender. The Zenz et al blender eliminated the large deadzones encountered in prior art bubbling-bed fluidized bed blenders by providing a fluidizing grid comprising a linear array of pyramidal-shaped hoppers, each hopper converging into a conically-shaped opening and a gas orifice for directing a flow of fluidizing gas downwardly into the bottom of the hopper. The elimination of the dead-zones enabled the Zenz et al blender to meet product homogeneity specifications for powder having an instantaneous flow function of about 4.0 or greater as measured by a Jenike-type flow factor tester. A compaction and build-up problem developed with fine, cohesive UO.sub.2 powders having an instantaneous flow function less than about 4.0 during blending in the Zenz et al blender. As used herein, the instantaneous flow function is the relationship between the unconfined yield strength and the consolidating pressures for the particles of powder being blended. The instantaneous flow function and the flow factor tester are more fully described in Bulletin No. 123, Utah Engineering Experimental Station, Storage and Flow of Solids by Andrew W. Jenike. Certain types of these fine and cohesive UO.sub.2 powders were found to deposit, compact and bridge in the hoppers of the Zenz et al blender. Since UO.sub.2 product homogeneity specifications require nearly ideal blending, this build-up in the hoppers of the Zenz et al blender prevented the blender from meeting product homogeneity specifications with UO.sub.2 powders having a flow factor less than about 4.0.
A general discussion of the design considerations involved in designing a prior art bubbling-bed fluidized bed blender including a consideration of particle properties, particle size, particle distribution, vessel geometry, superficial gas velocity, and circulation patterns is found in Fluidization and Particle Fluid Systems by Frederick A. Zenz and Donald F. Othmer, Reinhold Chemical Engineering Series, Reinhold Publishing Corporation, New York, 1960. Design considerations for possible grid designs are found in Fluidization by J. F. Davidson and D. Harrison, Academic Press, London, 1971.
The prior art also discloses spouting bed blenders, having a convergent hopper at the bottom of the blender with a plurality of mixing gas orifices disposed in a circular array at the bottom of the hopper, for example see British Pat. No. 900,242. Although the British patent shows orifices disposed at angles such that jets of mixing air are directed in a swirl pattern along the walls of the converging hopper, and in one embodiment the orifices are included in the closure member of a gate valve, there are differences between the device disclosed in the British patent and that of the present invention. These differences stem from the fact that the British device may be characterized as a spouting bed blender rather than a bubbling-bed fluidized bed blender. Spouting bed blenders employ gas mixing jets of sufficient power and duration to suspend and drive particles through the bed in a continuous stream that spouts from the top of the bed. In the British patent mixing is accomplished by pressure waves so dimensioned that the coarsest particles of the highest specific gravity in the mixture of particles in the mixing vessel are suspended upwardly. Fluidization in a bubbling-bed fluidized bed blender occurs at much lower mixing gas velocities. In fact, it is the lower mixing gas velocity of the bubbling-bed fluidized bed blender that makes it particularly suitable for blending UO.sub.2 powders. Spouting bed blenders have not been adopted for the blending of UO.sub.2 powders in particular because the violent action of gas jets sufficient to suspend the coarsest particles causes excessive loss of UO.sub.2 powder through entrainment with the mixing gas.
In bubbling-bed fluidized bed blenders the fluidizing grid (or gas distributor) at the bottom of the bed exerts a strong influence on the mixing process carried out in the bed. A coarse distributor produces high injection rates of gas at local points which leads to the channeling of gas within the bed. Channeling of gas causes deadzones of stagnant or unfluidized material. Since the gas mixing orifices disclosed in the British patent were intended to be used at much higher gas velocities than those necessary for bubbling-bed fluidized bed blending, channeling of gas and deadzones would occur in the spouting bed blender of the British patent at lower fluidizing gas velocities.